Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. The wireless communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an abnormal characteristic such as path loss, noise, fading due to multipath, an inter-symbol interference (ISI), the Doppler effect due to mobility of a user equipment, etc. Therefore, various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
Multiple Input Multiple Output (MIMO) is a technique for supporting reliable high-speed data services. The MIMO technique improves data transmission/reception efficiency by using multiple transmit (Tx) antennas and multiple receive (Rx) antennas. Examples of the MIMO technique include spatial multiplexing, transmit diversity, beamforming, etc.
Multiple receive antennas and multiple transmit antennas form a MIMO channel matrix. A rank can be found from the MIMO channel matrix. The rank is the number of spatial layers. The rank may also be defined as the number of spatial streams which can be transmitted at the same time by a transmitter. The rank is also referred to as a spatial multiplexing ratio. Assuming that the number of transmit antennas is Nt and the number of receive antennas is Nr, the rank R is R≦min{Nt, Nr}.
A wireless communication system requires a signal known to both a transmitter and a receiver to perform channel measurement, information demodulation, or the like. The signal known to both the transmitter and the receiver is referred to as a reference signal (RS). The RS may also be referred to as a pilot.
The receiver may estimate a channel between the transmitter and the receiver by using the RS, and may demodulate information by using the estimated channel. When a user equipment receives an RS transmitted by a base station, the user equipment may measure a channel by using the RS, and may feed back channel state information to the base station.
A signal transmitted from the transmitter experiences a channel corresponding to each transmit antenna or each spatial layer, and thus the RS may be transmitted for each transmit antenna or each spatial layer. If the RS is transmitted for each spatial layer, a plurality of RSs have to be transmitted such that the RSs do not cause spatial interference to each other. There is a need to consider an RS transmission method capable of minimizing spatial interference between RSs.
Accordingly, there is a need for a method and apparatus for efficiently transmitting signals in a wireless communication system.